1. Field of the Invention
This invention relates to Smear Suppressing CCD imager, and more particularly to a CCD imager capable of transferring signal charge at a high speed and at a high smear suppressing ratio.
2. Description of the Prior Art
Various CCD (charge coupled device) imagers are already known and have been put into practical use. Different exemplary ones of such conventional imagers are shown in FIGS. 5(A), 5(B) and 5(C). Referring to FIGS. 5(A) to 5(C), reference numeral 1 denotes a semiconductor substrate, and 2 an interlayer insulating film made of, for example, PSG (phosphor silicate glass) for isolating a transfer electrode from a first aluminum layer 1Al. Reference numeral 3 denotes a transfer electrode formed from a first or second polycrystalline silicon layer, and 4 a buffer wiring film formed from a third polycrystalline silicon layer for preventing a potential shift by the first aluminum layer 1Al. Reference numeral 5 denotes the first aluminum layer 1Al, and 6 a second aluminum layer 2Al.
The CCD imager shown in FIG. 5(A) is a most popular one of CCD imagers for use for the NTSC color television system which are currently available on the market, and in such CCD imager, the first layer aluminum film 5 is used only as a light intercepting film. In the CCD imager, a portion of the first layer aluminum film 5 which protrudes from the transfer electrode 3 is reduced in thickness, and also a corresponding portion of the interlayer insulating film 2 is reduced in thickness. Since the interlayer insulating film 2 is reduced in thickness in this manner, possible smears can be reduced. This is because, since external light is repetitively reflected by and between the semiconductor substrate 1 and the first layer aluminum film 5, the thinner the interlayer insulating film 2, the greater the number of times by which such external light advancing in the interlayer insulating film 2 is reflected and the greater the amount of attenuation of light admitted into the transfer electrode 3.
It is to be noted that, since the CCD imager is constructed to match the NTSC color television system, it is not necessary to minimize the resistance of the transfer electrode in order to assure a high speed transferring operation, and consequently, the CCD imager does not include a shunt wiring film therein.
Meanwhile, the CCD imager shown in FIG. 5(B) is constructed such that the first layer aluminum film (1Al) 5 is used as a shunt wiring film and also as a light intercepting film so as to achieve a high speed transferring operation. The first layer aluminum film 5 has, from a point of view of minimization in resistance, a great thickness at a portion thereof which protrudes from the transfer electrode 3 as well as the other portion thereof which remains above the transfer electrode 5. Consequently, also the interlayer insulating film 2 has a great thickness. Otherwise, possible damage to the semiconductor substrate 1 upon etching for patterning of the aluminum film 5 cannot be prevented with the interlayer insulating film 2. In short, in order to prevent possible damage to the semiconductor layer 1, it is necessary for the interlayer insulating film 2 to have a thickness which increases in proportion to the thickness of the aluminum film 5.
On the other hand, the CCD imager shown in FIG. 5(C) is constructed such that the first layer aluminum film 5 is used as a light intercepting film while the second layer aluminum film 6 is used as a shunt wiring film. The buffer wiring layer 4 formed from a third layer of polycrystalline silicon is utilized to establish electric connection between the aluminum film 6 serving as a shunt wiring film and the transfer electrode 3. Such CCD imager is disclosed, for example, in Japanese Patent Laid-Open Application No. 2-87574.
Such CCD imager permits to reduce the thickness of the light intercepting film formed form the first layer aluminum film 5 and form the interlayer insulating film 2 with a reduced thickness and can reduce possible smears by such reduction in thickness of the interlayer insulating film 2. Further, high speed transfer is made possible by forming the second layer aluminum film 6 with a comparatively great thickness to reduce the resistance of a route of a transfer controlling signal. Particularly, such CCD imager can cope with the HDTV (high definition television) system having up to two million picture elements.
The conventional CCD imagers described above individually have different drawbacks. In particular, the CCD imager shown in FIG. 5(A) is disadvantageous in that high speed transfer cannot be achieved because it has no shunt wiring film, and accordingly, while it can cope with the NTSC color television system, it cannot cope with the high definition television system. Meanwhile, in the CCD imager shown in FIG. 5(B), the first layer aluminum film 5 must be formed at every portion thereof with a great thickness, which makes a corresponding great offset or step. Accordingly, the CCD imager is disadvantageous in that it is not suitable for miniaturization and is difficult to contribute to realization of a television set of the high definition television system which employs a 2/3 inch optical system CCD imager. Besides, since the interlayer insulating film 2 has a great thickness, it is difficult to suppress possible smears.
On the contrary, the CCD imager shown in FIG. 5(C) is superior in that a high speed transferring operation can be achieved and possible smears can be suppressed. However, the CCD imager is disadvantageous in that a wiring configuration is very complicated. This is because, since the first layer aluminum film 5 is used only as a light intercepting film, electric connection between the transfer electrode 3 and the second layer aluminum film 6 must be established through an opening formed in the aluminum film 5 serving as a light intercepting film. Further, since the wiring configuration is complicated, further miniaturization of cells is difficult. Since considerable miniaturization of cells is required in order for a CCD imager of a 2/3 inch optical system and further of a 1/2 inch optical system to cope with the high definition television system, it is a significant problem that the wiring configuration is complicated.
By the way, one of CCD imagers is a CCD imager of the frame interline type. A frame interline type CCD imager includes an image section including a large number of light receiving elements arranged in a matrix and a plurality of vertical registers provided for the columns of the light receiving elements for transferring signal charge accumulated in the light receiving elements in the vertical directions, and a storage section including a plurality of vertical registers provided at vertically transferring ends of the image section for temporarily storing therein signal charge generated from the image section.
Such frame interline type CCD imager is required to increase the number of picture elements to two million or so in order to cope with the high definition television system. To this end, two horizontal registers must be included as disclosed, for example, in Japanese Patent Publication Application No. 1-13767. Further, in order to reduce the resistance of a transfer electrode made of polycrystalline silicon, a shunt wiring film made of aluminum must be connected in parallel to the transfer electrode as disclosed, for example, in Japanese Patent Application No. 1-188558.
FIG. 6 shows a wiring structure of an exemplary one of conventional frame interline type CCD imagers for two million picture elements. Referring to FIG. 6, the frame interline type CCD imager shown includes a semiconductor substrate 21 and a pair of transfer electrodes 22a and 22b made of polycrystalline silicon. The transfer electrode 22a is made of first layer polycrystalline silicon 1Poly while the other transfer electrode 22b is made of second layer polycrystalline silicon 2Poly. The frame interline type CCD imager further includes a buffer wiring layer 23 made of third layer polycrystalline silicon 3Poly for electric the connection between the transfer electrode 22a and a shunt wiring layer 26. The buffer wiring layer 23 is provided to prevent a possible potential shift by first layer aluminum 1Al. An imager including a buffer wiring layer is disclosed in Japanese Patent Laid-Open Application No. 2-87574 as mentioned hereinabove. Originally, such third layer polycrystalline silicon 3Poly is a polycrystalline silicon layer necessitated for the formation of a gate provided between two horizontal registers to alternatively transmit signal charge from a storage section to a first or second one of the horizontal register in which such signal charge should subsequently be transferred horizontally. However, third layer polycrystalline silicon 3Poly forms, in an image section and the storage section, a buffer wiring layer which also effects a relaying or repeating operation.
The CCD imager further includes an interlayer insulating film 24, a light intercepting film 25 made of the first layer aluminum 1Al, and a shunt wiring film 25 made of second layer aluminum 2Al.
The CCD imager has, in either of the image section and the storage section, such a wiring structure as shown in FIG. 6.
However, while there is no specific problem if such wiring structure as shown in FIG. 6 is employed for the image section, there are some problems in employing, for the storage section, such a wiring structure as shown in FIG. 6 wherein a light intercepting film is formed from the first layer aluminum film 1Al while a shunt wiring film is formed from the second layer aluminum film 2Al. Thus, it is a first problem that a further high light intercepting characteristic cannot be obtained.
In particular, since a low speed shift of signal charge takes place in the storage section, a higher light intercepting performance than the OPB (optical black) of the storage section in which a high speed frame shift is performed is required. However, since the storage section has a same wiring structure as the image section, only a similar degree of light intercepting performance can be obtained. More particularly, it cannot be prevented completely that external light may be admitted into the inside while it is repetitively reflected by and between the first layer aluminum film (1Al) 25 and the second layer aluminum film (2Al) 26 as seen from FIG. 6 until it is introduced into a vertical transfer register to cause smears.
Secondly, where the light intercepting film is formed from the first layer aluminum film (1Al) and the shunt wiring film is formed from the second layer aluminum film (2Al), the opening 27 must be formed in the light intercepting film 25 such that the shunt wiring film 26 may be contacted with the buffer (transit) wiring film 23 through the opening 27, and it is a problem that the configuration is complicated. In particular, in the image section, since it includes the light receiving section and the vertical transfer section, even if the configuration is complicated, it still is necessary to form a light intercepting film from the first layer aluminum film (1Al) 25 and form a shunt wiring film from the second layer aluminum film (2Al) 26. However, in the storage section, there is no necessity of employing such complicated configuration as described above since it has no light receiving section but only has the vertical transfer section. Nevertheless, in the conventional CCD imager, the storage section is constructed in a complicated configuration similarly to the image section. This is not preferable because it will cause deterioration in reliability and reduction in yield in manufacture of CCD imagers.